In a conventional tensioner, a plunger, protruding from a housing, exerts a force on the back of a pivoted tensioner lever having a shoe in sliding contact with the slack (return) run of a chain in order to maintain tension in the chain. The plunger of the tensioner engages the lever at a location remote from the pivot axis of the lever.
A chain tensioner which we have developed is depicted in FIG. 8. As shown in FIG. 8, the tensioner 500 comprises a plunger 520, which protrudes retractably from a plunger-receiving hole 511 in a housing 510. A biasing spring 530, biases the plunger 520 in the protruding direction. Wedge-shaped cam chips 540, which slide in sloped guide grooves 512 formed at the open front end of the plunger-receiving hole 511, engage racks 521 formed on opposite sides of the plunger 520. At least two cam chips are provided. A cam-biasing spring 550 loosely fitting on the plunger 520, biases the wedge-shaped cam chips 540, pushing them into the plunger-receiving hole 511. The outer end of the cam-biasing spring 550 bears against an inwardly projecting flange of a spacer 560 secured to the front end of the housing 510.
As a chain becomes elongated during use, the chain tensioner 500 controls backlash by allowing the plunger 520 to proceed sequentially, one tooth at a time in the projecting direction. In this way, the tensioner maintains appropriate chain tension, and prevents foreign noise upon starting of the engine, and whistling sounds which occur as a result of excess chain tension.
In the design of the chain tensioner 500, the backlash distance was not particularly considered, foreign noises were generated due to wobbling of the chain upon starting the engine, and hewing sounds were generated during excess tensioning of the chain. These problems often arise due to the fact that the setting of the backlash distance is not matched to the individual engines. When the backlash distance is relatively large, the wobbling sounds due to wobbling of the chain are liable to occur upon starting of the engine, and when the backlash distance is relatively small, the plunger 520 protrudes excessively, and hewing sounds due to excess tensioning of the chain are likely to occur.
The backlash distance also changes before and after the cam chips 540 step from one tooth of the rack 521 to another. When the backlash distance, just before the cam chips 540 step from one tooth to another, is set to a maximum, and the backlash distance just after the cam step from one tooth to another is set at a minimum, the balance between the maximum and minimum backlash distances has a significant effect on the above-mentioned foreign noise.
Adjusting the backlash distance by changing the design of the tooth length or height of the racks on the plunger 520 was tried in order to solve these problems. However, wear occurred in the cam chips 540 and in the rack 521, and chipping of the teeth was experienced due to poor tooth strength.
Restrictions in the design of the rack limited the adjustability of the backlash distance to an extremely narrow range so that the tensioner could not be applied to a wide variety of engines. As a result wobbling sounds from the chain, and whistling sounds due to excess protrusion of the plunger occurred.
Accordingly, objects of the invention include the solution to the above-described problems, and the provision of a chain tensioner which can prevent wobbling noises of the chain upon starting of and engine and whistling sounds due to excess tensioning of the chain, which can maintain appropriate chain tension, and in which the amount of backlash can be set to accommodate a wide variety of engines.